Coal for use in a thermal power plant or an iron mill is typically stored in a form of a pile in an outdoor yard. The coal stored in such a way may generate heat through a reaction with oxygen in air, leading to spontaneous ignition. In particular, low-grade coal is porous and high in oxidation reactivity, and therefore easily generates heat. To measure this, water is typically sprinkled to the pile to prevent the spontaneous ignition. This measure however requires periodic sprinkling. Hence, there is a demand for a method for efficiently preventing the spontaneous ignition.
Under such a circumstance, there have been developed techniques for preventing the spontaneous ignition of the coal pile, such as a technique of covering a pile surface with resin or the like (see Japanese Unexamined Patent Application Publication No. Hei5 (1993)-230480 and Japanese Unexamined Patent Application Publication No. 2000-297288), and a technique of spraying a surfactant containing a free radical scavenger or an oxygen trapping compound (see Japanese Unexamined Patent Application Publication No. 2001-164254). Each of such techniques however requires the resin, the free radical scavenger, or the like, and may cause an increase in cost.
In addition, there has been developed a method for producing upgraded coal from low-grade coal (porous coal) that is high in water content and low in calorific power (see Japanese Unexamined Patent Application Publication No. Hei7 (1995)-7-233383). In this method, first, porous coal is pulverized into particles, and then mixed with a mixed oil including a heavy oil content and a solvent oil content to produce a material slurry. Subsequently, the material slurry is preheated and heated to accelerate dehydration of the porous coal, and to allow the mixed oil to penetrate into pores of the porous coal, so that a dehydrated slurry is yielded. Subsequently, the upgraded porous coal and the mixed oil are separated from the dehydrated slurry, and then the upgraded porous coal is dried (dehydrated). The dried, upgraded porous coal is cooled and molded if desired. According to such a method, the water content of the porous coal is decreased, and the heavy oil adheres onto the inside of each pore of the porous coal, so that an upgraded coal high in calorific power is produced.
The upgraded coal produced by such a method is molded into briquettes from the viewpoint of workability including transporting operation and of suppressing dusting. When the briquettes are stored in a form of a pile, the pile is high in gas permeability since the briquettes have the same shape. Hence, when a coal having relatively high oxidation reactivity is piled, or when the pile has a great height, temperature of the pile increases in a relatively short time. For such an upgraded coal, therefore, there is a particular need for a storing technique that allows spontaneous ignition to be reduced.